System and method for heating material employing oversize waveguide applicator

ABSTRACT

A microwave heating applicator is provided in the form of a rectangular-type waveguide having dimensions a and b transverse of power flow through the guide. The b dimension is maintained less than one-half the free-space wavelength of the excitation frequency, lambda o; and the dimension a is enlarged beyond lambda o thereby reducing the attenuation constant for the applicator and the heating rate of the material being treated. The microwave power is transmitted through a treating zone in a first direction; and the electric field intensity varies, transverse of the direction of power flow, from a minimum at one side of the treating zone to a maximum at the center of the zone and back to a minimum at the other side of the treating zone. These sides of the treating zone define the dimension a; and the material is passed through the treating zone either along the direction of power flow or perpendicular to the direction of power flow, depending upon the application.

United States Patent [72] Inventor Ray M. Johnson Danville, Calif.

[21] Appl. No. 816,500

[22] Filed Apr. 16, 1969 [45] Patented Jan. 4, 1972 [73] Assignee Cryodry Corporation San Ramon, Calif.

[54] SYSTEM AND METHOD FOR HEATING MATERIAL EMPLOYING OVERSIZE WAVEGUIDE OTHER REFERENCES Foundations for Microwave Engineering, Collins, 1966, pages 95- 100 Primary Examiner.l. V. Truhe Assistant Examiner-L. H. Bender AttorneysCarl C. Batz and Dawson, Tilton, Fallon &

Lungmus ABSTRACT: A microwave heating applicator is provided in the form of a rectangular-type waveguide having dimensions a and b transverse of power flow through the guide. The b dimension is maintained less than one-half the free-space wavelength of the excitation frequency, A,,; and the dimension a is enlarged beyond A, thereby reducing the attenuation constant for the applicator and the heating rate of the material being treated. The microwave power is transmitted through a treating zone in a first direction; and the electric field intensity varies, transverse of the direction of power flow, from a minimum at one side of the treating zone to a maximum at the center of the zone and back to a minimum at the other side of the treating zone. These sides of the treating zone define the dimension a; and the material is passed through the treating zone either along the direction of power flow or perpendicular to the direction of power flow, depending upon the application.

PATENTED JAN 4 I972 Jason,

SYSTEM AND METHOD FOR HEATING MATERIAL EMPLOYING OVERSIZE WAVEGUIDE APPLICATOR BACKGROUND OF THE INVENTION The present invention relates to a system for heating, cooking, or drying material with microwave energy. In particular, the invention relates to a system for heating with microwave energy wherein the applicator in which the material is treated is a wave guide.

An early development in the field of microwave or electronic cooking was the construction of a box providing a cavity of generally rectangular shape and designed for heating food. The relative dimensions of wavelength and cavity at a given frequency are such that the cavity is several wavelengths on a side. Food material is placed in the cavity, the door closed, and the oven turned on. The oven is designed, by means of mode stirrers and reflectors, such that the heat generated in the oven is generally independent of the position of the material being heated. This type of oven is sometimes referred to as a batch-type oven since the material is heated or cooked in a batch; and the oven must be shut down for putting food into and taking food from the oven. Sometimes these ovens are referred to as multimode cavities since they are designed to excite many different modes of the microwave energy within the cavity; and reflectors and stirrers are usually provided in an effort to evenly distribute the multimode microwave energy.

A continuous-type microwave oven was developed which feeds microwave power into an elongated tunnel through a series of slit openings spaced longitudinally of the tunnel. These tunnels serve the same function as multimode cavities since a number of different modes are excited as the microwave energy enters the tunnel. A continuous belt conveys the material being heated through the tunnel; and energy-absorbing devices (in the form of traps cooled by circulating liquid) are provided at each end of the tunnel adjacent the entrance and exit openings for preventing the escape of excess radiation and for absorbing excess microwave power to prevent damaging the microwave system. The openings for coupling the microwave energy from the source to the interior of the cavity cause higher losses and lower the Q (i.e., the ratio of stored energy to dissipative energy) of the cavity. These energy losses cannot be recouped.

Another development included the use of a waveguide folded into a serpentine arrangement (sometimes referred to as a meander" system) and adapted so that the product passes through aligned slots in a number of such folds. Energy is piped into one end of the waveguide and the energy level decreases along the length of the waveguide which finally terminates in a water load. This decrease in power along the direction of power flow is especially noticeable for lossy products. The rate of decrease in power with respect to 2 (which, by convention, is the direction of power flow through a waveguide) is defined as the attenuation constant of a waveguide applicator.

A waveguide refers to a hollow pipe or conduit of generally closed cross section for propagating and channeling electromagnetic energy in the microwave region. The term waveguide is generic, as used herein, since it includes both the conduit which couples power from the source to the applicator as well as the applicator itself which provides the heating chamber" or "treating zone, as it is sometimes referred to. The direction in which the energy propagates is referred to as the z direction. A rectangular waveguide is a waveguide of generally rectangular cross section, although it need not necessarily be a true rectangle. The lowest order mode (i.e., TE mode) of propagation of microwave energy through a waveguide is characterized in that the electric field lines extend between two opposing wall surfaces (sometimes called the broadwalls"), and that these field lines are generally parallel to each other and perpendicular to the flow of microwave energy (or power). The Ycoordinate is taken to be the direction along which these field lines extend; and the x coordinate extends perpendicular to the y direction and to the flow of power. Hence, the x and y coordinates are orthogonal to each other and define a plane perpendicular to the flow of power through a waveguide.

if the cross section of the waveguide is rectangular, the distance separating the broadwalls (i.e., taken along the y coordinate) is b; and the distance separating the other sidewalls (i.e., the width of the broadwalls and taken along the x coordinate) is a.

TE modes of operation refer to transverse electric field vectors; and TM modes refer to transverse magnetic field vectors.

The TE mode of operation describes the electric field within the waveguide wherein: (a) there is no component of the electric field which extends in the z direction; (b) there are m nodes (minima or maxima) in the electric field intensity profile along the x direction; and (c) there are n nodes in the electric field intensity profile along the Ydirection Thus, the TE mode of operation in a waveguide refers to a condition wherein the electric field vector extends perpendicularly between the broadwalls (in the Y direction), and there is substantially no electric field in the z or 1: directions. It will be appreciated that the foregoing explanations, which have been for purposes of illustration, are theoretical. ln practice, these concepts are useful design tools only; and they are not to be taken as rigorous definitions of structure, operation or result. The free-space wavelength of the excitation frequency h is the length of a period of the excitation frequency as it would exist if the wave were propagated through free space. The b dimension may be held at a value less than one-half the free-space wavelength of the excitation frequency to suppress the propagation of all TM modes and all TE modes where n is an integer greater than zero. Further, the a dimension may be maintained to be greater than one-half of the free-space wavelength and less than one free-space wavelength to suppress TE modes where m is greater than one.

In the TE mode, the intensity of the electric field varies according to a sinusoidal function from a minimum at one side of the treating zone through a maximum at the transverse center of the broadwall (i.e., x=a/2), and back to a minimum at the opposite side of the treating zone. Thus, for a given input power, the electric field intensity at the transverse center of the treating zone is at a maximum. in the previously described meander systems, the material being treated is conducted through the waveguide at a slot formed at this midpoint and elongated in the direction of power flow. As will be explained in greater detail herein, the electric field strength at the slot position is reduced by increasing either dimension, a or b. A lower limit on electric field strength is imposed by the TE mode restrictions on a and b.

If the attenuation constant is very high and the application is, for example, the drying of wallboard having a substantial width extending in the direction of power flow, then very much more power will be dissipated at the input end of each of the folded sections of the applicator than will be dissipated at the output end. Under these circumstances, only one edge of the material would be sufficiently treated. That is to say, considering just one pair of slots centered on opposing broadwalis and adapted to admit passage of the wallboard, the slots are elongated in the direction of power flow (the z direction) so that the wallboard is moved transverse of the z direction and power flows from one side edge of the wallboard to the other. Since the high moisture content of the wallboard causes it to be lossy, a good deal more power will be dissipated in the first edge of the material than will be dissipated in the second edge thereby causing uneven drying. It is true that this undesirable effect is somewhat compensated for in a member system having a number of folds because in the first pass, the power moves in one direction across the material; and in the second pass, the power moves in the opposite direction. However, for lossy material, this compensation is not very effective; and uneven heating of the material nevertheless results.

There are many advantages in both design and efficiency in using a waveguide as a heat applicator over multimode cavities. In a waveguide applicator in which a traveling wave is excited and power flows through the applicator, the modes of excitation and propagation are more controllable in the sense that a system can be designed to propagate only those modes which are desired. This leads to the ability to predetermine the dissipation of energy in the material being treated, and to design against the escape of energy by radiation for those modes alone.

Further, as a practical matter, most sources of microwave power include a magnetron tube which is designed to generate microwave energy over a very narrow frequency band; and although the frequency of the magnetron may vary slightly, its output power can drop markedly for such frequency variations. There is a complex interrelationship between the frequency of oscillation of a magnetron and the load which it feeds. Thus, if the load varies even slightly, the frequency of oscillation of a magnetron may shift and its output power may vary appreciably. If an applicator has a high O (which is desirable to enhance efficiency), then the load which the magnetron sees should be kept substantially constant even though the amount of material being treated may vary appreciably. The maintenance of a constant load as seen by the magnetron keeps the operation of the magnetron within a very narrow frequency band-thus preserving its operating life because of low reflected power and insuring operation at a much higher efficiency.

By making the transverse width of the broadwall oversize, the power intensity profile becomes more spread out and the power density reduces for a fixed input power. Further, the attenuation constant and heating rate are reduced to cause a more even heating of the material along the direction of power flow. As used herein, oversize means that the width of the broadwalls is greater than a free-space wavelength of the excitation frequency.

This result, it will be appreciated is advantageous not only in applications wherein the material is fed through center slots in the broadwall, but also in applications wherein the material passes between the broadwalls along the direction of power flow and not through them. Further, it is desirable in a batchtype oven for achieving a more even heating.

In applications in which it is desired to provide a slotted broadwall, the waveguide coupling power from the source to the applicator is symmetrical about a plane passing through the center lines of the broadwalls in order to inhibit the excitation of TE,,,,, modes where m is an even integer. For those modes where m is an odd integer, there will be no excessive radiation through a center slot in the broadwalls.

Further, this concept includes an alternative embodiment wherein the broadwall is slotted and tapered to provide increased uniformity in heating rate in the direction of power flow. That is, the attenuation rate is increased 3 as the power available decreases to provide a more unifonn amount of energy deposits at all positions along the direction of power flow.

Other features and advantages of the present invention will be apparent to persons skilled in the art from the following detailed description accompanied by the attached drawing.

THE DRAWING FIG. 1 is a perspective view of a meander waveguide system according to the present invention;

FIG. 2 is a closeup view of a portion of the system of FIG. 1 illustrating adjacent sections of the applicator.

FIG. 3 is a reduced front elevation view of a typical section of the applicator system of FIG. 1;

FIGS. 4-4A illustrate the advantages of the present invention in use with a waveguide applicator wherein the material being treated between the broadwalls;

FIG. 5 is a graph illustrating the reduction in attenuation constant and heating rate in the direction of power which are accomplished by the present invention; and

FIG. 6 is a reduced front elevation view of an alternative embodiment wherein the broadwall is tapered.

DETAILED DESCRIPTION It is believed that the theoretical aspects of the present invention will be better understood with the inventive concept first explained in the setting of particular applications. Thcrefore, before discussing the theoretical aspects of the invention, the application of the invention in a serpentine or meander system will be explained. Referring then, to FIGS. 1-2, input microwave power is received from a source of microwave energy 10 along an input waveguide 10a which, in this illustration, assumes a generally horizontal disposition. For purposes explained in greater detail herein, the waveguide input feed system is symmetrical about a plane parallel to the direction of power flow and passing through the center lines of opposing broadwalls.

All of the waveguide elements described herein are made of an electrically conductive material, such as aluminum. The input waveguide section 10a may assume a taper or draft to enlarge its cross-sectional area defined by a peripheral flange 11. The flange 11 is connected to a corresponding peripheral flange 12 located at the input end of a waveguide elbow section 13 which makes a bend.

The broadwalls of the waveguide elements illustrated in FIGS. 1 and 2 extend in a generally vertical plane, one such broadwall being designated 13a for the input elbow l3 seen in FIG. 1. For this same elbow, the opposing sidewalls are designated 13b and respectively. The transverse cross section of each of the illustrated waveguide elements (that is, in a plane perpendicular to the flow of power when excited in the TE mode) is substantially rectangular in shape.

Power is coupled from the input elbow 13 to a plurality of waveguide applicator sections designated respectively 14, 15, 16 and 17. Each applicator section provides a heating chamber or treating zone. It will be appreciated that any number of such sections may be arranged through which the material being treated may pass. A coupling section 18 connects the elbow 13 to the first applicator section 14; and adjacent applicator sections are connected to each other by means of shaped coupling members designated 19, 20 and 21 respectively. Power flows in opposite directions in adjacent ones of the applicator sections or folds. The output of the final applicator section 17 is connected to a similar U-shaped section 22 which may go to a subsequent applicator section or may serve to couple power to a termination which may be provided with a waterload for receiving and dissipating all remaining energy not already spent in heating the material being treated.

The material being treated is designated 24; and in this application, it is in the form of a flat sheet of lossy material such as veneer paneling or wallboard. The sheet material 24 passes through the waveguide applicator sections 14-17 by means of slots centered in each of the broadwalls of these applicator sections and extending in the direction of power flow. As can be seen in FIG. 2, for adjacent applicator sections 15 and 16, a slot 26 is formed in a first broadwall 27 of the applicator section 15; and the slot 26 is centered on the horizontal centerline in the broadwall 27 and extends in the direction of power flow. A similar slot (hidden from view in FIG. 2) is formed in the opposing broadwall 28 of applicator section 15. These slots are horizontally aligned for passing the material 24 completely through the applicator section 15 transverse of the broadwalls 27 and 28. Similarly, a slot 30 is formed in a broadwall 31 of the applicator sections 16 and a slot 32 is formed in the opposing broadwall 33 of this section.

The material 24 is both driven and supported by a plurality of pairs of vertically spaced drive rollers. A first pair of rollers is generally designated by reference numeral 35 in FIG. 1; and it is located prior to the entrance of the material into the, waveguide applicator section 14. Subsequent pairs of drive rollers are located between adjacent of the applicator sections, the one between applicator sections 15 and 16 comprising an upper roller 37 and a lower roller 38 as seen in FIG. 2. Means are provided for adjusting the spacing of the rollers and the tension with which the pairs of rollers grip the material being processed.

Still referring to FIG. 2, the placement of the U-shaped coupling section 20 is illustrated in phantom. Referring now in particular to the applicator section 16, the width of the broadwall 31 is designated by the dimensional arrow and the width of the sidewalls of the applicator section 16 are defined by the dimensional arrow 1) as shown therein.

Turning now to FIG. 3, the applicator section 16 is shown in front elevation view; and it will be observed that it includes peripheral side flanges 40 and 41 for connecting respectively to corresponding flanges on the U-shaped connecting members 20 and 21. For simplicity in construction and maintenance as well as cleaning the interior of the applicator section 16, it may be provided in left and right half sections which are flanged peripherally about its midsection but which do not interfere with passage through the elongated slots 30 and 32.

FIG. l-3 illustrate an embodiment wherein the material being processed is passed perpendicularly through the opposing broadwalls of each applicator section and wherein the cross-sectional area of the waveguide is maintained substantially constant, but the invention is not so limited. The invention is, however, particularly useful in such applications for, as will be discussed in greater detail within, the inventive system pennits the inclusion of the elongated slots midway of the broadwalls and extending in the direction of power flow without substantial radiation of microwave energy through these slots.

In an alternative embodiment, the material being treated may be passed entirely within the waveguide applicator section-in which case the material (if it is a sheet material) would lie in a plane parallel to the broadwalls. Referring to FIG. 4, the cross section of a rectangular applicator section is shown in line diagram from with the upper and lower broadwalls being designated 43 and 44 and the opposing sidewalls 45 and 46 respectively. The material being treated is designated by reference numeral 47. Again, the transverse width of the broadwalls 43 and 44 is indicated by the dimension a; and the transverse width of the sidewalls 45 and 46 is indicated by the transverse dimension b.

In this embodiment, the material may be conveyed along (i.e., either with or against) the direction of power flow or it may remain stationary. The same effects of a more even heating rate are achieved.

In FIG. 6, there is shown a waveguide section similar to that shown in FIG. 3 wherein the broadwall is designated 31' and the center slot 30. It will be appreciated that the broadwalls of the waveguide in FIG. 6 are tapered, becoming similar in width as one proceeds in the direction of power flow. Thus, the cross-sectional area of the guide reduces in the direction in which power flows for reasons to be discussed presently.

In slotted waveguide structures of the type shown in FIGS. 1-3 (that is, the meander or serpentine arrangements) the dimensions a and b may be constrained such that only the TE mode is allowed to propagate. That is, the dimension 0 (the width of the broadwalls) is everywhere between one-half A and a full A and the dimension b is everywhere less than or equal to one-half A Since the frequencies available for commercial heating by microwave are restricted by government regulation and, to some extent, by the operating frequencies of available magnetrons, these restraints on the dimensions a and b for a given frequency impose an upper limit on the size of the waveguide.

The electric field intensity for the TB in a plane transverse of the flow of power is a sinusoidal function, having a value of zero at each of the sidewalls and a maximum value midway between the sidewalls. In the illustration of FIG. 4, for example, wherein the dimension [1 is one-half the free-space wavelength of the excitation frequency, the dimension a (which is defined by the vertical dashed lines) defines the upper limit for the transverse width of the broadwalls. The equation for the electric field strength or intensity in the TE mode is given below as equation (I). The x direction is the horizontal direction in FIG. 4, the y direction is the vertical direction in FIG, 4, and the z direction is perpendicular to the plane of the page of FIG. 4, (that is, in the direction of flow of power).

E =E,, sin (1rx/a) (I) The power transmitted through a waveguide in the TB mode is the integral of the sealer product of the Poynting vector and the differential cross-sectional area of the guide (is. a x b). Theoretically, the power is calculated according to eq.

n 1h' where E,, the electric field intensity at FG/Z (that is, the maximum electric field intensity).

A =free space wavelength v: characteristic impedance of free space From equation (2), it can be seen that for a constant input power (which is the usual case) the maximum electric field intensity, E,,, varies inversely with the transverse area of the wave guide; and for a constant b dimension and a/)\ greater than I, the maximum electric field strength is approximately inversely proportional to the square root of the dimension a. In other words, as the dimension a is increased, the maximum electric field intensity decreases, and vice versa.

The power distribution in a plane transversely of the direction of flow of the power is proportional to the product E, sin ('rrx/a). This distribution is diagrammatically illustrated by the curve designated 48 in FIG. 4A for the case in which the dimension A is maintained to be less than the freespace wavelength of the excitation frequency (as with conventional rectangular waveguides excited in the TE mode).

The square of the electric field strength on center may be normalized with respect to the output power according to eq.

From eq. (4), it can be seen that by enlarging dimension a greater than A E becomes smaller and smaller. Thus, the power distribution curve changes from that of the graph 48 to the one illustrated by the curve 49 in FIG. 4A as the transverse width of the broadwalls changes from the dimension a to the dimension a. In FIG. 4, the horizontal axis indicates transverse position along the broadwall (x direction) and the vertical axis is power. It can be seen from FIG. 4A that with increase in the dimension a, the power on centerline decreases from P, to the value P While increasing the dimension a greater than the freespace wavelength of the excitation frequency clearly decreases the square of the normalized electric field strength, it also permits propagation of TE modes where m is an integer greater than one. These other modes, if strongly excited, may cause appreciable radiation to occur from the longitudinal slot in the embodiment of FIGS. 13. It will be noticed however, that the dimension b is maintained to be less than one-half the free-space wavelength of the excitation frequency. Further, as long as the waveguide applicator is excited symmetrically relative to a plane parallel to the direction of power flow and passing through the longitudinal centerline of the broadwalls of the applicator sections, TE,,,,, modes wherein m is an even integer will not be excited. The TE,,,,, modes wherein m is an odd number greater than one, although they may be excited, will not radiate through the longitudinal slot at x=a/2 because the magnetic field in the z direction, H, is zero at x=a/2.

Thus, if care is maintained in the excitation of the oversize waveguide at Fa/Z, the TE, modes with m an even integer are not excited with a significant amount of energy; and leakage of microwave power cut of the slot will be constrained to acceptable limits.

The heating rate at the first applicator section 14 for the serpentine arrangement will be reduced as compared with waveguide applicators wherein a is less than h In addition, however, the change in heating rate with 2 will also be significantly reduced so that the heating will be more evenly spread out as the material is being processed. That is, the heating from one edge of the wallboard to the other will be more uniform; and subsequent sections will have a heating rate nearly equal to that of the preceding section if the waveguide is oversize.

This effect of decreasing the attenuation constant in the direction of power flow is considered to be an important advantage of the present invention; and it is graphically illustrated in FIG. wherein the vertical coordinate is power density and the horizontal coordinate is the z direction. FIG. 5 therefore illustrates power density distribution at the plane x=al2 extending along the direction of flow of power; and the curve designated by reference numeral 50 illustrates this for a waveguide applicator with a less than M. With the dimension as increased beyond the free-space wavelength of the excitation frequency, the power density distribution curve changes to that illustrated by the curve 51 wherein it can be seen that the initial power density decreases from P, to P and the attenuation becomes more uniform. It will be appreciated that the attenuation (i.e. dP/dz) varies with the first derivative of the curves 50 and 51.

This effect of achieving a more uniform heating rate in the direction of power flow is important not only in the serpentine arrangement wherein the heating rate for subsequent ones of the applicator section becomes more uniform, but also in the arrangement illustrated in FIG. 4 wherein the material being traced is conveyed parallel to and between the broadwalls. By thus'causing the heating rate to be more uniform in the z direction, extreme temperatures at the location at which the material is introduced into the applicator are avoided. The advantage of providing a longitudinal slot extending in the direction of elongation of the broadwall and centered about the line x=a/2, is also important in applications wherein a liquid such as fat or moisture is rendered as the material is heated.

It will be apparent to persons skilled in the art that the inventive principles described herein may also be used in combination with elements of the waveguide applicator such system described in the above-identified, copending application such as the means for circulating air through the waveguide, the waterload which terminates the waveguide downstream in the direction of flow of power from the applicator sections, or the rejection filters designed to reject energy at the locations of input and egress of the material for the embodiment illustrated in FIG. 4.

Finally from eq. (4), it will be appreciated that the normalized electric field intensity may be increased in the direction in which power flows by maintaining b as a constant and by decreasing the width, 11, of the broadwall to achieve a taper as illustrated in FIG. 6. This taper will increase the field intensity at the slot even though the overall power decreases by attenuation.

Other modifications may be made to the structure disclosed and equivalent structure substituted for that described without departing from the inventive principles; and it is, therefore, intended that all such modifications and equivalents be covered as they are embraced within the spirit and scope of the invention.

lclaim:

l. A system for applying microwave energy to material comprising a source of microwave energy at a wavelength, A

waveguide applicator means having transverse dimensions, a and b in a plane lying perpendicular to the flow of power therethrough, wherein b is less than about one-half A, and a is greater than A, and means coupling said source to said applicator means to excite the same.

2. The system of claim I wherein said dimension, a, defines the transverse width of a pair of opposing broadwalls extending in the direction of power flow and wherein at least one of said broadwalls defines a slot centered at about the transverse center thereof and extending in the direction of power flow.

3. The system of claim 1 wherein said dimension, a, defines the transverse width of a pair of opposing broadwalls extending in the direction of power flow and wherein each of said broadwalls defines a slot centered at about the transverse center thereof and extending in the direction of power flow and further comprising means for moving said material through said slots transverse of the direction of power flow.

4. The system of claim 3 wherein said applicator means is formed into a plurality of folds, each fold being an applicator section, and wherein all of said broadwalls define slots centered at about the transverse center thereof and extending in the direction of power flow, all of said slots being aligned to permit the passage of said material therethrough.

5. The system of claim 1 wherein said waveguide applicator means is defined by a first pair of opposing broadwalls extending in the direction of power flow and having a transverse dimension equal to a, and a second pair of opposing sidewalls extending in the direction of power flow and having a transverse dimension equal to b, and wherein said material is disposed between said broadwalls over an extended area.

6. A system for applying heat to sheet material comprising source means generating microwave energy, waveguide means excited by said source principally in the TE mode whereby power flows along said waveguide means, said waveguide means having a rectangular transverse cross section providing a pair of opposing broadwalls separated by a distance of less than about one-half the free-space wavelength of the excitation frequency, and a pair of opposing sidewalls separated by a distance greater than the free-space wavelength of said excitation frequency, said opposing broadwalls defining a pair of slots extending along the centerline thereof in the direction of flow of power for receiving said material, and means supporting said material for moving the same through said pair of slots transverse of the flow of power through said waveguide means thereby to diminish the attenuation constant through said material in the direction of power flow.

7. The system of claim 6 wherein said waveguide means forms a plurality of separate applicator sections with power flowing in opposite directions in adjacent sections and corresponding broadwalls being parallel, the broadwalls of each applicator section defining a pair of slots extending along the centerline of said broadwalls in the direction of power flow, all of said slots being aligned to receive said material.

8. They system of claim 7 further comprising coupling waveguide means receiving power from said source and coupling the same to said waveguide means, said coupling waveguide means being symmetrical about a plane parallel to the direction of power flow and passing through the centers of said pair of slots to prevent excitation of TE, modes where m is an even integer.

9. The system of claim 7 wherein the transverse width of said pair of opposing broadwalls is progressively diminished to define a taper while maintaining the separation between said broadwalls less than one-half the free-space wavelength of the excitation frequency, said taper being symmetrical about a plane parallel to the direction of power flow and passing 11. The system of claim 10 wherein said opposing broadwalls define parallel sides of said waveguide means separated by a distance less than about one-half of the free-space wavelength of said excitation frequency.

12. A method of heating a material comprising: generating a microwave electric field having a free-space wavelength of predetermined dimension; confining said microwave field in a treating zone such that substantially the entire electric field vector lines thereof lie in a plane and the intensity of said field in said plane varies from a minimum at a first side of said zone through a maximum at a central location in said zone and thence to a minimum at a second side of said zone; transmitting power of said field in a direction perpendicular of said plane; and applying said microwave field to said material in said treating zone to heat the same while maintaining the spacing between said first and second minimum locations defining the sides of said treating zone greater than the free-space wavelength of said microwave field at least over a portion of said treating zone.

13. The method of claim 12 further comprising the step of maintaining the dimension of said treating zone extending transverse of the direction of power flow and transverse of the direction of said field lines less than one-half the free-space wavelength of said excitation frequency.

14. A method of heating a material comprising: transmitting in a first direction through a treating zone microwave power provided by an electric field excited at a predetermined frequency and having field lines extending substantially entirely in a plane perpendicular to said first direction, the intensity of said field in said plane varying from a minimum at a first side of said treating zone through a maximum at an intermediate location in said treating zone and thence to said minimum as a second side of said treating zone; moving said material through said treating zone in a direction perpendicular to said first direction and through said maximum level of field intensity whereby said microwave power is transmitted through said material transverse of the direction of movement of said material through said treating zone; and maintaining the distance between said first and second sides of said treating zone at which said field intensity is at said minimum greater than the free-space wavelength of said excitation frequency over at least a portion of said treating zone.

15. The method of claim 14 wherein said material is generally planar and said field lines are substantially parallel to each other in said treating zone and wherein said step of moving said material comprises passing said material through said treating zone in a direction parallel to said field lines and to said direction of transmission of said microwave power.

16. The method of claim 14 further comprising converging the spacing of said first and second sides of said treating zone in the direction of power flow.

17. The method of claim 14 further comprising the step of confining said field lines transverse of their direction of extension within less than one-half of the free-space wavelength of the excitation frequency in said treating zone.

18. A method of heating a material comprising: transmitting microwave power through a treating zone in a first direction, said power being provided by an electric field excited at a predetermined frequency and having field lines substantially parallel to each other and within a plane perpendicular to said first direction, the intensity of said field in said plane along a line transverse of said field lines varying from a minimum at a first side of said treating zone through a maximum at an intermediate location in said treating zone and thence to a minimum at a second side of said treating zone; moving said material through said treating zone along said first direction; and maintaining said first and second sides of said treating zone at a distance greater than the free-space wavelength of said excitation frequency over at least a portion of said treating zone.

19. The method of claim 18 further comprising confining said field along the direction of said field lines to less than onehalf wavelength of the excitation frequency. 

1. A system for applying microwave energy to material comprising a source of microwave energy at a wavelength, lambda o, waveguide applicator means having transverse dimensions, a and b, in a plane lying perpendicular to the flow of power therethrough, wherein b is less than about one-half lambda o and a is greater than lambda o and means coupling said source to said applicator means to excite the same.
 2. The system of claim 1 wherein said dimension, a, defines the transverse width of a pair of opposing broadwalls extending in the direction of power flow and wherein at least one of said broadwalls defines a slot centered at about the transverse center thereof and extending in the direction of power flow.
 3. The system of claim 1 wherein said dimension, a, defines the transverse width of a pair of opposing broadwalls extending in the direction of power flow and wherein each of said broadwalls defines a slot centered at about the transverse center thereof and extending in the direction of power flow and further comprising means for moving said material through said slots transverse of the direction of power flow.
 4. The system of claim 3 wherein said applicator means is formed into a plurality of folds, each fold being an applicator section, and wherein all of said broadwalls define slots centered at about the transverse center thereof and extending in the direction of power flow, all of said slots being aligned to permit the passage of said material therethrough.
 5. The system of claim 1 wherein said waveguide applicator means is defined by a first pair of opposing broadwalls extending in the direction of power flow and having a transverse dimension equal to a, and a second pair of opposing sidewalls extending in the direction of power flow and having a transverse dimension equal to b, and wherein said material is disposed between said broadwalls over an extended area.
 6. A system for applying heat to sheet material comprising source means generating microwave energy, waveguide means excited by said source principally in the TE10 mode whereby power flows along said waveguide means, said waveguide means having a rectangular transverse cross section providing a pair of opposing broadwalls separated by a distance of less than about one-half the free-space wavelength of the excitation frequency, and a pair of opposing sidewalls separated by a distance greater than the free-space wavelength of said excitation frequency, said opposing broadwalls defining a pair of slots extending along the centerline thereof in the direction of flow of power for receiving said material, and means supporting said material for moving the same through said pair of slots transverse of the flow of power through said waveguide means thereby to diminish the attenuation constant through said material in the direction of power flow.
 7. The system of claim 6 wherein said waveguide means forms a plurality of separate applicator sections with power flowing in opposite directions in adjacent sections and corresponding broadwalls being parallel, the broadwalls of each applicator section defining a pair of slots extending along the centerline of said broadwalls in the direction of power flow, all of said slots being aligned to receive said material.
 8. The system of claim 7 further comprising coupling waveguide means receiving power from said source and coupling the same to said waveguide means, said coupling waveguide means being symmetrical about a plane parallel to the direction of power flow and passing through the centers of said pair of slots to prevent excitation of TEmo modes where m is an even integer.
 9. The system of claim 7 wherein the transverse width of said pair of opposing broadwalls is progressively diminished to define a taper while maintaining the separation between said broadwalls less than one-half the free-space wavelength of the excitation frequency, said taper being symmetrical about a plane parallel to the direction of power flow and passing through the centerlines of said pair of slots.
 10. In a microwave heating system, the combination of a waveguide of rectangular type cross section excited in the TE10 mode and provided with a pair of opposing broadwalls having a width transverse of the direction of power flow greater than the free-space wavelength of the excitation frequency, and material in said waveguide between said broadwalls for dissipating microwave energy therein.
 11. The system of claim 10 wherein said opposing broadwalls define parallel sides of said waveguide means separated by a distance less than about one-half of the free-space wavelength of said excitation frequency.
 12. A method of heating a matErial comprising: generating a microwave electric field having a free-space wavelength of predetermined dimension; confining said microwave field in a treating zone such that substantially the entire electric field vector lines thereof lie in a plane and the intensity of said field in said plane varies from a minimum at a first side of said zone through a maximum at a central location in said zone and thence to a minimum at a second side of said zone; transmitting power of said field in a direction perpendicular of said plane; and applying said microwave field to said material in said treating zone to heat the same while maintaining the spacing between said first and second minimum locations defining the sides of said treating zone greater than the free-space wavelength of said microwave field at least over a portion of said treating zone.
 13. The method of claim 12 further comprising the step of maintaining the dimension of said treating zone extending transverse of the direction of power flow and transverse of the direction of said field lines less than one-half the free-space wavelength of said excitation frequency.
 14. A method of heating a material comprising: transmitting in a first direction through a treating zone microwave power provided by an electric field excited at a predetermined frequency and having field lines extending substantially entirely in a plane perpendicular to said first direction, the intensity of said field in said plane varying from a minimum at a first side of said treating zone through a maximum at an intermediate location in said treating zone and thence to said minimum as a second side of said treating zone; moving said material through said treating zone in a direction perpendicular to said first direction and through said maximum level of field intensity whereby said microwave power is transmitted through said material transverse of the direction of movement of said material through said treating zone; and maintaining the distance between said first and second sides of said treating zone at which said field intensity is at said minimum greater than the free-space wavelength of said excitation frequency over at least a portion of said treating zone.
 15. The method of claim 14 wherein said material is generally planar and said field lines are substantially parallel to each other in said treating zone and wherein said step of moving said material comprises passing said material through said treating zone in a direction parallel to said field lines and to said direction of transmission of said microwave power.
 16. The method of claim 14 further comprising converging the spacing of said first and second sides of said treating zone in the direction of power flow.
 17. The method of claim 14 further comprising the step of confining said field lines transverse of their direction of extension within less than one-half of the free-space wavelength of the excitation frequency in said treating zone.
 18. A method of heating a material comprising: transmitting microwave power through a treating zone in a first direction, said power being provided by an electric field excited at a predetermined frequency and having field lines substantially parallel to each other and within a plane perpendicular to said first direction, the intensity of said field in said plane along a line transverse of said field lines varying from a minimum at a first side of said treating zone through a maximum at an intermediate location in said treating zone and thence to a minimum at a second side of said treating zone; moving said material through said treating zone along said first direction; and maintaining said first and second sides of said treating zone at a distance greater than the free-space wavelength of said excitation frequency over at least a portion of said treating zone.
 19. The method of claim 18 further comprising confining said field along the direction of said field lines to less than one-half wavelength of the excitation frequency. 